The present invention relates to a high frequency plasma power supply and an automatic impedance matching device and more particularly to a system for supplying power to a plasma chamber in a semiconductor processing apparatus using high frequency plasma CVD or high frequency plasma etching treatment.
When power is to be supplied to a reaction chamber (plasma chamber) in high frequency plasma CVD process or high frequency plasma etching treatment, matching has to be made between the load impedance and the high frequency power supply impedance.
The load impedance of the chamber is determined by the magnitude of power to be supplied to the chamber (external condition) and various internal conditions, and high frequency power is supplied between the electrodes after starting at zero level it has undergone dark current level and dark current sweep leading to ignition level. In this case, since the high frequency plasma power supply receives reflective power from the chamber, a simple increase in applied power would result in the power value of the reflective waves reaching about 100% of the applied power, leading to the danger of the power supply device being damaged. Therefore, it has been common practice to resort to the shutdown method which is very complicated, consisting in controlling the traveling wave power value of the high frequency plasma power supply to ensure that the reflective wave power value is 10-20% less than the rated output while effecting impedance matching, thus increasing the power to the value necessary for the plasma reaction process for the whole chamber.
More particularly, during the process in which raw gas introduced into the chamber is converted into plasma by the power from a high frequency plasma power supply, the gas plasma load impedance sharply changes at the time of ignition. Further, since it also changes according to the flow rate, pressure, temperature of the raw gas, differences between the impedance of the high frequency plasma power supply (which impedance is usually taken to be 50.OMEGA., the same value as that of coaxial cables) and the impedance in the chamber irregularly occur. For this reason, use is made of an automatic impedance matching device for an impedance matching circuit associated with power supply to the chamber, and it usually takes about 3-10 seconds before impedance matching for the driving system is attained. During this matching operation, 100% of the power is not consumed in the chamber, producing power reflection which accounts for the occurrence of power being fed back to the high frequency plasma power supply as described above.
In the conventional high frequency plasma power supply, to prevent damage to the final stage device (transistors, particularly FETs), use is generally made of the shutdown method to suppress the traveling wave power on the supply side so as to hold reflective wave power at less than 10-20% of the rated output if such reflective wave power is produced during mismatching of the plasma impedance. Since this method is a feedback control, i.e., an electrical loop control, it has the drawback that it sometimes fails to cope with a sharp change in impedance in the chamber, leading to damage to the final stage device of the high frequency plasma power supply.
In semiconductor producing factories, effective measures have been sought for to lower the high frequent breakdown in the power supply device, said high frequent breakdown has been a great obstacle to full automation.
In the shutdown method used to elevate the power to the value necessary for plasma process while controlling the traveling wave power value on the supply side, since there is a great difference in pre-ignition impedance and post-ignition impedance on the condition that the initial power necessary for plasma ignition is at least 10-20% of the rated output, the period of matching operation of the automatic impedance matching device ranging from the time when power supply is started to the time when the power reaches the value necessary for the process is relatively too long for smooth implementation of the recent process. And another drawback is that since this period is not definite, reproducibility is poor. Further, among the drawbacks in semiconductor treatment due to long plasma reaction time at a power value lower than the rated output is the problem that in CVD process, uniformity of film quality and reproducibility of film thickness are poor while in thin film etching process, overetching and underetching frequently occur owing to the poor reproducibility of applied power.